Continuation-Local Storage ( Typescript )

This is a fork of cls-hooked implemented with Typescript.

Support for node < 10 has been removed, and it will no longer use a process global. In addition, an optional dependency supporting weak references has been made available.

A little history of "AsyncWrap/async_hooks" and its incarnations

1. First implementation was called AsyncListener in node v0.11 but was removed from core prior to Nodejs v0.12
2. Second implementation called AsyncWrap, async-wrap or async_wrap was included to Nodejs v0.12.
3. Third implementation and offically Node-eps accepted AsyncHooks (async_hooks) API was included in Nodejs v8. :)

Continuation-local storage works like thread-local storage in threaded programming, but is based on chains of Node-style callbacks instead of threads. The standard Node convention of functions calling functions is very similar to something called "continuation-passing style" in functional programming, and the name comes from the way this module allows you to set and get values that are scoped to the lifetime of these chains of function calls.

Suppose you're writing a module that fetches a user and adds it to a session before calling a function passed in by a user to continue execution:

// setup.ts
import { createNamespace, Namespace } from 'cls-ts';
import * as db from './lib/db';

const session: Namespace = createNamespace('my session');

const start = (options, next) => {
  db.fetchUserById(options.id, (error, user) => {
    if (error) {
      return next(error);
    }

    session.set('user', user);

    next();
  });
};

Later on in the process of turning that user's data into an HTML page, you call another function (maybe defined in another module entirely) that wants to fetch the value you set earlier:

// send_response.ts

import { getNamespace, Namespace } from 'cls-ts';
import * as render from './lib/render'

const session: Namespace = getNamespace('my session');

const finish = response => {
  const user = session.get('user');
  render({user: user}).pipe(response);
};

When you set values in continuation-local storage, those values are accessible until all functions called from the original function – synchronously or asynchronously – have finished executing. This includes callbacks passed to process.nextTick and the timer functions (setImmediate, setTimeout, and setInterval), as well as callbacks passed to asynchronous functions that call native functions (such as those exported from the fs, dns, zlib and crypto modules).

A simple rule of thumb is anywhere where you might have set a property on the request or response objects in an HTTP handler, you can (and should) now use continuation-local storage. This API is designed to allow you extend the scope of a variable across a sequence of function calls, but with values specific to each sequence of calls.

Values are grouped into namespaces, created with createNamespace(). Sets of function calls are grouped together by calling them within the function passed to .run() on the namespace object. Calls to .run() can be nested, and each nested context this creates has its own copy of the set of values from the parent context. When a function is making multiple asynchronous calls, this allows each child call to get, set, and pass along its own context without overwriting the parent's.

A simple, annotated example of how this nesting behaves:

import { createNamespace, Namespace, Context } from 'cls-ts';
const writer: Namespace = createNamespace('writer');

const requestHandler = () => {
  writer.run(function(outer: Context) {
    // writer.get('value') returns 0
    // outer.value is 0
    writer.set('value', 1);
    // writer.get('value') returns 1
    // outer.value is 1
    process.nextTick(function() {
      // writer.get('value') returns 1
      // outer.value is 1
      writer.run(function(inner: Context) {
        // writer.get('value') returns 1
        // outer.value is 1
        // inner.value is 1
        writer.set('value', 2);
        // writer.get('value') returns 2
        // outer.value is 1
        // inner.value is 2
      });
    });
  });

  setTimeout(function() {
    // runs with the default context, because nested contexts have ended
    console.log(writer.get('value')); // prints 0
  }, 1000);
};

writer.run(function () {
  writer.set('value', 0);

  requestHandler();
});

Weak References

Version 0.2.0 adds an optional dependency, weak-napi. When available, namespaces and contexts bound to Event Emitters will be weak, allowing them to be garbage collected if the corresponding Namespace is destroyed.

Docs

cls.createNamespace(name)

• return: {Namespace}

Each application wanting to use continuation-local values should create its own namespace. Reading from (or, more significantly, writing to) namespaces that don't belong to you is a faux pas.

cls.getNamespace(name)

• return: {Namespace}

Look up an existing namespace.

cls.destroyNamespace(name)

Dispose of an existing namespace. WARNING: be sure to dispose of any references to destroyed namespaces in your old code, as contexts associated with them will no longer be propagated.

cls.reset()

Completely reset all continuation-local storage namespaces. WARNING: while this will stop the propagation of values in any existing namespaces, if there are remaining references to those namespaces in code, the associated storage will still be reachable, even though the associated state is no longer being updated. Make sure you clean up any references to destroyed namespaces yourself.

cls.fromException

When an exception is thrown from a function wrapped with a namespace, through run or bind and associated methods, the active context is attached to the thrown error using a private symbol. This method will attempt to retrieve the context attached to an error.

Class: Namespace

Application-specific namespaces group values local to the set of functions whose calls originate from a callback passed to namespace.run() or namespace.bind().

namespace.active

• return: the currently active context on a namespace

namespace.set(key, value)

• return: value

Set a value on the current continuation context. Must be set within an active continuation chain started with namespace.run() or namespace.bind().

namespace.get(key)

• return: the requested value, or undefined

Look up a value on the current continuation context. Recursively searches from the innermost to outermost nested continuation context for a value associated with a given key. Must be set within an active continuation chain started with namespace.run() or namespace.bind().

namespace.run(callback)

• return: the context associated with that callback

Create a new context on which values can be set or read. Run all the functions that are called (either directly, or indirectly through asynchronous functions that take callbacks themselves) from the provided callback within the scope of that namespace. The new context is passed as an argument to the callback when it's called.

namespace.runAndReturn(callback)

• return: the return value of the callback

Create a new context on which values can be set or read. Run all the functions that are called (either directly, or indirectly through asynchronous functions that take callbacks themselves) from the provided callback within the scope of that namespace. The new context is passed as an argument to the callback when it's called.

Same as namespace.run() but returns the return value of the callback rather than the context.

namespace.runPromise(promiseGenerator)

• return: the generated {Promise} with cls then's attached

Create a new context on which values can be set or read. Run all the functions that are called (either directly, or indirectly through further asynchronous functions that take callbacks themselves) from the provided promise generator within the scope of that namespace. The {Promise} generated by the promiseGenerator only needs to be Promise-like, non-null, with both a then and catch function defined. The new context is passed as an argument to the promise generator when it's called.

namespace.bind(callback, [context])

• return: a callback wrapped up in a context closure

Bind a function to the specified namespace. Works analogously to Function.bind() or domain.bind(). If context is omitted, it will default to the currently active context in the namespace, or create a new context if none is currently defined.

namespace.bindEmitter(emitter)

Bind an EventEmitter to a namespace. Operates similarly to domain.add, with a less generic name and the additional caveat that unlike domains, namespaces never implicitly bind EventEmitters to themselves when they're created within the context of an active namespace.

The most likely time you'd want to use this is when you're using Express or Connect and want to make sure your middleware execution plays nice with CLS, or are doing other things with HTTP listeners:

http.createServer(function (req, res) {
  writer.bindEmitter(req);
  writer.bindEmitter(res);

  // do other stuff, some of which is asynchronous
});

namespace.createContext()

• return: a context cloned from the currently active context

Use this with namespace.bind(), if you want to have a fresh context at invocation time, as opposed to binding time:

function doSomething(p) {
  console.log("%s = %s", p, ns.get(p));
}

function bindLater(callback) {
  return writer.bind(callback, writer.createContext());
}

setInterval(function () {
  var bound = bindLater(doSomething);
  bound('test');
}, 100);

Context

A context is a plain object created using the enclosing context as its prototype.

Copyright & License

See LICENSE for the details of the BSD 2-clause "simplified" license used by continuation-local-storage. This package was developed in 2019-2020 as a fork of cls-hooked written by Jeff Lewis.